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USB 2 Maximum Cable Length

zazal

OSNN One Post Wonder
#1
My backup device is a USB 2.0 250 MB Iomega Hard Drive.I'd like to put
it in my garage, requiring about 20 to 25 feet of wire. The official
USB specs say 5 meters max; if further use a hub/repeater.

I'm wondering how realistic the 5 meter spec is. I recall that parallel
printer cables were supposed to be limited to 10 feet or so; 75 foot
cables worked just fine. Same thing with the specs for RS-232 cables.

Does anyone have any real world experience with long USB 2.0 cables?
I'm thinking of building mine using unshielded CAT5 cable, one twisted
pair for power, another for data. Wise? foolish?
 
Last edited by a moderator:

fitz

Woah.. I'm still here?
Staff member
Political User
#2
personally, I wouldn't do it. As it's your backup device, would you want to risk data loss and/or data corruption caused by signal degradation onto a backup device?

A printer is one thing to try to stretch the distance limit.. a Hard drive and backup data is entirely different.

Go buy a USB hub, put it somewhere in between and get two 5m cables.

http://www.usb.org/developers/usbfaq/#cab1
1. Why are there cable length limits, and what are they?
A: The cable length was limited by a cable delay spec of 26ns to allow for reflections to settle at the transmitter before the next bit was sent. Since USB uses source termination and voltage-mode drivers, this has to be the case, otherwise reflections can pile up and blow the driver. This does not mean the line voltage has fully settled by the end of the bit; with worst-case undertermination. However, there's been enough damping by the end of the bit that the reflection amplitude has been reduced to manageable levels. The low speed cable length was limited to 18ns to keep transmission line effects from impacting low speed signals.

2. I want to build a cable longer than 5 meters, why won't this work?
A: Even if you violated the spec, it literally wouldn't get you very far. Assuming worst-case delay times, a full speed device at the bottom of 5 hubs and cables has a timeout margin of 280ps. Reducing this margin to 0ps would only give you an extra 5cm, which is hardly worth the trouble.
 

zeke_mo

(value not set)
Staff member
Political User
#3
My backup device is a USB 2.0 250 MB Iomega Hard Drive.I'd like to put
it in my garage, requiring about 20 to 25 feet of wire. The official
USB specs say 5 meters max; if further use a hub/repeater.

I'm wondering how realistic the 5 meter spec is. I recall that parallel
printer cables were supposed to be limited to 10 feet or so; 75 foot
cables worked just fine. Same thing with the specs for RS-232 cables.

Does anyone have any real world experience with long USB 2.0 cables?
I'm thinking of building mine using unshielded CAT5 cable, one twisted
pair for power, another for data. Wise? foolish?
250MB? Might be time to upgrade
 

gonaads

Beware the G-Man
Political User
#4
Don't push the limit when it comes to cable length specs. These are set so you don't get data corruption. It's better to use a hub/repeater and be totally safe especially if you data is important.

Now my curiosity is getting the better of me. Why do you want to put the backup drive in your garage so far away? External drives are small and take up little room/space. Why not have it close by?
 

Tuffgong4

The Donger Need Food!!!!
Political User
#6
is it a temperature thing??? do you want the hard drive to stay cool?

either way when it's data or even sensitive data the shorter the cable the better.

Also if you've just purchased the backup drive see if you can buy an external hard drive with networking instead, (NAS)
 
#7
You don't put computer equipment in a garage, regardless of cable length. Heat, cold, humidity, dust, roaches, ants and other vermin crawling around inside of it will trash the device.
 

X-Istence

*
Political User
#8
You don't put computer equipment in a garage, regardless of cable length. Heat, cold, humidity, dust, roaches, ants and other vermin crawling around inside of it will trash the device.
I absolutely agree. I used to have a machine I kept downstairs in the basement, and when I finally had to move it with me to Arizona, I took the case apart after it having been online for a good year and a half and it was FILLED with dust. I was surprised the fans even kept moving in that thing (good thing they did :p).

Took a good hour to vacuum it out, and get all the dust out of the heatsink.
 

Heeter

Overclocked Like A Mother
#11
Hmmm,

I have a printer on a custom made 50 foot USB cable. It's no HD, but I haven't had any problems with it. Maybe printers are easier on data transfer than Hd's.


Heeter
 

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